What about 6x? I find for my dd, that we have to buy jeans with the elastic adjuster inside the waistband. You might try those, too.
But, your dd might be like me. Even with petite sizing, I have to get every pair of pants shortened. *%!*!# my short legs!

I'm having the same problem with my 5 year old DD too, OP. It seems like everything this season is in the skinny jean camp and everything is too tight! My DD is short and muscular...she's not overweight by any stretch but she does have muscular legs, etc. She wears school uniform jumpers to school, so my situation has not been an emergency yet and I've gotten by with leggings so far.

I'm thinking about trying consignment shops where the hand-me-down jeans may still have some room in them. All I need is one decent pair for cold weather outings. I feel your frustration.

I had that problem with dd when she was 5. I was SO frustrated. Somehow we muddled through, mostly by rolling up the legs, and cinching the waists of larger pants. I bought her a pair of boy's jeans that she loved. She wanted to wear all boy's pants after that. I did get her a couple of pair of boy jean shorts over the summer, too. They are just so much more comfortable. I got the jeans at Kohl's, Levi's, I think. And the shorts were the $3-$5 kind from Walmart.

She mostly wore two dresses with leggings from gymboree last year. I got them both from ebay.

I was very, very pleased to discover this year (at 6), she has lengthened, but not gotten bigger around, and all the stuff I bought last year fits her SO cute this year. (She never was chubby, even, btw.)

And, oddly enough...I noticed that ds, who is 4.5, is chunking up in much the same way dd did. Maybe it's a 4-5 yo thing. :)